A quantum key distribution system is configured with a transmitter, a receiver, and an optical fiber that connects the transmitter and the receiver. The transmitter transmits photons to the receiver via the optical fiber (a quantum communication channel). After that, the transmitter and the receiver exchange control information with each other, and share encryption keys in a confidential fashion. This technology is implemented using the technology generally referred to as quantum key distribution (QKD).
In quantum key distribution, the behavior of the photons follows the uncertainty principle, which is the fundamental principle of quantum mechanics that tapping leads to changes in the state. Because of such a property, when photons transmitted by a transmitter are tapped by an eavesdropper in a quantum communication channel, the state of the photons undergoes a change and the receiver that receives the photons becomes able to get to know that the photons have been tapped by an eavesdropper.
The encryption keys generated in a quantum key distribution system are used by various applications that perform cryptographic communication. Usually, if many encryption keys are generated, then there is a proportional increase in the encryption strength and communicable time of the cryptographic communication. Hence, it is desired that encryption keys having a large size be generated in large numbers with a high frequency.
Meanwhile, it is necessary that the transmission of photons in quantum key distribution is performed while ensuring synchronization between the transmitter and the receiver. Hence, if the operations are temporarily stopped, then the temporal cost of restarting the operations is high. Therefore, it is not desirable to stop the transmission of photons during quantum key distribution. However, if generation of encryption keys is carried on in concert with the transmission of photons, then it may lead to an increase in the processing load of the transmitter and the receiver that generate the encryption keys, or it may lead to unnecessary generation of the encryption keys, or it may place burden on the amount of available space in the storage used in storing the encryption keys.